Photolithography is used to produce semiconductor integrated circuits (ICs). Such a process uses photographic techniques to impart patterns for the construction of ICs on wafers. Due to the extremely small size of these intricate circuits, the manufacturing process must be excessively monitored to ensure that contaminating particles are not present. A particle may be inadvertently introduced to the process, often times becoming sandwiched between the wafer and its retaining vacuum platform. The intruding particle causes a region of the wafer to be subject to improper focusing by the photographing unit.
Improper focusing on such a wafer region causes what is known as a focus spot. Such spots are recognizable by human inspectors and are a major problem for chip makers. Focus spots can affect perhaps 4% of all batches. There is a need in the art for an automated system for focus spot detection. Such a system could inspect a batch of wafers soon after their production, thus saving the time lag experienced as human inspectors more slowly examine the wafers.
Such a system could save expense as well as time. Frequently, a surface contaminant will remain on the vacuum platform and will cause focus spots in several wafers at the same location. By detecting the appearance of a focus spot soon after the batch has been completed, engineers can be notified to clean the vacuum platform before the next batch of wafers is introduced.
The detection method must also be accurate. False focus spot detection is detrimental to the manufacturing process because such false flagging of batches will cause added work for human inspectors, and will cause unnecessary interruption of the manufacturing process for machine inspection and cleaning.